This invention relates to abrasive tools containing metal coated superabrasives in a vitrified bond. The tools are particularly useful in the high performance grinding of ceramics.
When first introduced, synthetic diamond, including synthetic diamond coated with metals such as nickel or copper, was used in a variety of resin bonded and metal bonded abrasive tools. Early versions of such tools are described in U.S. Pat. No. 3,925,035 and U.S. Pat. No. 3,984,214. In metal bonded wheels, metal coated diamond is indicated for less severe grinding conditions. The thin nickel or copper coating on the diamond apparently improves adhesion and heat transfer of the synthetic diamond in abrasive wheels.
Due to the difficulty in bonding diamond to the glass of the vitrified bond matrix and the high temperature firing cycles needed to cure vitrified bonded abrasive tools, synthetic diamond and CBN were not used in vitrified bonds when these grains were first introduced. More recently, as described, e.g., in U.S. Pat. No. 4,157,897 and U.S. Pat. No. 4,951,427, metal coated diamond and cubic boron nitride (CBN) have been suggested for use in vitrified bonded abrasive tools.
Other protective coatings have been suggested. To protect CBN from oxidation during firing of vitrified bonded tools, a coating of a thin inner layer of a reactive glass or ceramic and, optionally, an outer layer of a vitreous material may be coated onto the abrasive grain as described in U.S. Pat. No. 5,300,129. The metal or glass coatings prevent the reaction between CBN and the alkali metal oxides of the bond and creation of gaseous byproduct and bloating at the grain/bond interface during firing of the bond. The bloating may cause loss of shape of the abrasive tool.
Many of the improvements in vitreous bonded tools since the introduction of metal coated superabrasive relate to the grinding of steel and other metals, particularly the grinding of metals using CBN abrasive. Little has been reported on the use of coated diamond in vitreous bonded tools for use in grinding ceramics and other non-ferrous materials.
In U.S. Pat. No. 4,157,897, copper, silver, nickel, cobalt, molybdenum and alloys thereof are suggested for coating diamond for use in vitrified bonded tools containing 23 to 53%, on a bond volume basis, of crystalline (flake) graphite for the dry grinding of cemented carbide articles. The tools of the invention include grinding wheels containing no more than 10% total porosity and hones containing no more than 15% porosity. The metal cladding is an optional element of the abrasive tools and no measurable improvement is reported in grinding performance on cemented tungsten carbide of the nickel coated diamond relative to uncoated diamond.
It has now been discovered that metal coated superabrasive will significantly improve the grinding performance of vitreous bonded abrasive tools containing at least 10% porosity and about 2 to 20% solid lubricant, such as graphite. Grinding performance improvements include a higher ratio of material removal rate relative to abrasive wheel wear rate (G-ratio), reduced surface waviness on the workpiece. These improvements have been particularly beneficial in the surface grinding of the sapphire windows commonly used in retail store scanning devices.